Definition for Epistasis
From Biology Forums Dictionary
1) A gene interaction in which the alleles of one gene mask the expression of the alleles of another gene.
2) Epistasis is a phenomenon that consists of the effect of one gene being dependent on the presence of one or more 'modifier genes' (genetic background). Similarly, epistatic mutations have different effects in combination than individually.
3) A form of gene interaction in which one gene prevents or masks the expression of a second gene.